Effect of speed conversion mode on the stability of mixed-flow pump during variable speed process

The speed conversion strategy has a significant impact on the stability of variable speed operation of the mixed flow pump. This study takes the mixed flow pump as the research object, collects the displacement signals of the shaft vibration and the pressure pulsation signals inside the pump under different speed conversion modes based on a synchronous acquisition system, and analyzes the influence of speed conversion modes on the two. The results show that the position of the shaft center is inconsistent with before the start of speed conversion at the end of speed conversion, and the offset distance of the quartic power speed conversion is the smallest, which is 0.075 mm. When the speed conversion to the quartic power, the vortex motion of the shaft vibration is the most stable compared to the other two speed conversions. When the quartic power speed conversion, the ratio of S_(p-p) to ∆D is only greater than 0.33 at the end of the speed conversion. The maximum two-dimensional joint kurtosis values for the three speed conversion modse are 5.67 (quarter power), 5.61 (linear), and 5.4 (quartic power), respectively. Among the three speed conversion modes, the proportion of positive joint kurtosis of shaft vibration is 70% (quarter power), 44% (linear), and 4% (quartic power), respectively. The quartic power speed conversion mode can effectively improve the mechanical impact of shaft vibration. The main frequency of shaft vibration under different speed conversion modes is the rotational frequency f_n, and the proportion of high amplitude region f_n is 87.5%, 62.5%, and 12.5% in order of the index of its speed conversion mode, from small to large. The high amplitude and high-order harmonic frequencies of shaft vibration are mainly concentrated in 2f_n among the three speed conversion modes. During the speed conversion process, phase resonance causes the amplitude of higher-order harmonics of pressure pulsation inside the impeller to be stronger than f_BPF. And the proportion of high amplitude areas in 12f_n under the three speed conversion modes is 75%, 50%, and 5% in order of their speed conversion mode indices, from small to large. During the process of speed conversion, the main frequency of shaft vibration velocity still exists in the time-frequency of pressure pulsation inside the pump, indicating that shaft vibration has a certain impact on pressure pulsation inside the pump Finally, the improved TOPSIS method was used to quantitatively evaluate the operational stability of the mixed-flow pump, and the impact of speed conversion modes on the operational stability of the mixed flow pump was explored. The results showed that as the speed increased, the operational stability of the mixed flow pump gradually deteriorated. The stability index of the quartic power speed conversion is the smallest, while the stability index of the quarter power speed conversion is the largest. Under the premise of only considering shaft vibration and pressure pulsation inside the pump, the mixed flow pump has the best stability during the quartic power speed change process. The research results provide a certain reference for improving the stability of mixed flow pump during speed conversion process.